As described in Non-Patent Document 1, a critical dimension SEM which is a type of Scanning Electron Microscope (SEM) that is specialized for semiconductors is used for management of pattern dimensions in a semiconductor production process. The principle of the critical dimension SEM is shown in FIG. 2. A primary electron beam emitted from an electron gun 010 is narrowed and converged by convergence lenses 011 and a specimen is two-dimensionally scanned with the electron beam through a deflector 012 (scanning coil).
A detector 014 captures secondary electrons emitted from the specimen 020 as a result of irradiating the specimen 020 with the electron beam focused on the surface of the specimen 020 by an objective lens 013, with the result that an electron beam image is obtained. Because more secondary electrons are emitted from the edges of a pattern, the portions corresponding to the edges of the pattern are seen brighter in the electron beam image. A series of operations is carried out by a control unit 015.
The magnification of a scan image can be changed arbitrarily by a ratio between a scan width (fixed) on CRT and a scan width (variable) of the electron beam on the specimen. In FIG. 2, if the magnification of SEM is denoted by M and a pattern dimension on screen I, an actual dimension S is represented by I/M. In the critical dimension SEM, a position at which a dimension is to be measured is specified on a scan image and, then, the dimension is measured by executing a calculation depending on the applied magnification, using a signal waveform of that position.
Although different types of methods of automatic dimension measurement using signal waveforms have been proposed, a “threshold method” which is a typical method is shown in FIG. 3. As mentioned above, more secondary electors are emitted from the edges of a pattern. The portions having larger amplitudes of signal, corresponding to the left and right edges of the pattern, are to be denoted by a left white band (left WB) and a right white band (right WB), respectively. The threshold method obtains Max and Min values for the left and right WBs, respectively, calculates a threshold value from those values, detects a position at which a signal waveform intersects the threshold value as an edge portion, and determines a distance between left and right edges to be a dimension (CD) value. The threshold value in FIG. 3 can be determined arbitrarily by a user.
A sequence of an automatic dimension measurement, which is generally applied, is shown in FIG. 4. A wafer is loaded (step 101), a stage is moved in proximity to a dimension measurement location (step 102), and an image is captured at a low magnification on the order of 10,000 times (step 103). An accurate position of the dimension measurement location is determined by pattern recognition that uses a registered image as a template (step 104). By limiting a primary electron beam scan range to a narrower region around the determined position (step 105), an image is captured at a high magnification on the order of 150,000 times (step 106) and dimensions are measured (step 108).
The above operation that changes the image capturing position by changing the primary electron beam scan position without stage movement is to be referred to as an image shift. The reason why, after an image is captured at a low magnification, an image shift is performed and an image is captured again at a high magnification, without starting with capturing an image at a high magnification, is that it is generally hard to set a pattern to be measured covered in the region of a high magnification image due to insufficient precision of stopping the stage in place.
With further microfabrication of semiconductor patterns, measurement precision requirements of critical dimension (CD) SEMs become more strict year by year. In addition to requirements in terms of individual tool reproducibility of measurement for CD SEMs, existing heretofore, it becomes a significant challenge to reduce a difference between dimensions measured by respective devices (tool to tool matching), because a plurality of CD SEMs are often used together in view of a relationship between the processing capacity of a single CD SEM and the amount of semiconductor production.